The invention relates to a locking device for a camshaft adjuster including a vane rotor mounted on the camshaft and disposed in a drive housing such that the angular position of the vane rotor relative to the drive housing is adjustable.
Camshaft adjusters are used in valve-controlled internal combustion engines in order to vary the rotational angle or to bring about a phase offset of the camshaft relative to the crankshaft or other camshafts. The opening times of the exhaust and inlet valves are thereby advanced or retarded in relation to the top dead center position of a piston, in order to optimize the charge cycle of the combustion chambers or cylinders as a function of the operating point of the internal combustion engine at any given time.
DE 101 27 168 A1 discloses a camshaft adjuster having a vane rotor. The vane rotor is rotationally locked to the camshaft and is supported so that it can rotate to a limited extent in a drive part, which is generally driven by the crankshaft of the internal combustion engine by way of a toothed belt. With its vanes the vane rotor meshes in gaps in the drive part which are formed by projections extending radially inwards. The projections define the maximum rotational angle though which the vane rotor can be turned relative to the drive part. Between the vanes and the projections hydraulic chambers are formed, which are controlled by a control valve, so that the position of the vane rotor relative to the drive part is determined by the pressure ratios of the pressure chambers situated in a peripheral direction on both sides of a vane.
When the internal combustion engine is shut down springs set the vane rotor to one of the two limit positions. Upon starting of the internal combustion engine the rotating cams of the camshaft exert alternating reaction forces in the direction of rotation, which lead to advancing or retarding torques. If only a slight pressure, if any, has built up in the pressure chambers, the reaction forces overcome the spring forces, so that the vanes in contact with the radial projections lift off and move to the opposite radial projection. When they strike against these, or when they impinge on the contact face again when the reaction forces are reversed, they generate annoying noises. For this reason the vane rotor is generally locked in relation to the drive part by a locking bolt, which is supported either in the vane rotor or in the drive part so that it is axially or radially displaceable, and extends in a locking well in the other part. A spring presses the locking bolt into the locking well. The locking bolt is hydraulically released by the control valve first admitting fluid to a pressure chamber when the internal combustion engine is started. When the pressure in the pressure chamber becomes greater than the pressure of the locking spring, the locking bolt is pushed out of the locking well and the camshaft adjuster is operable.
In the locked limit position of the vane rotor, the camshaft must be held at an angle of rotation, relative to the position of the camshaft, which is conducive to the startup of the internal combustion engine. For this reason an inlet valve camshaft adjuster is generally locked in a retarded limit position and an exhaust valve camshaft adjuster in an advanced limit position. In order to allow a larger adjustment range, particularly for the inlet valves, the vane rotor is locked between the two limit positions for starting of the internal combustion engine. After starting, with the locking bolt in the release position a control valve controls or regulates the rotational angle according to recorded operating performance characteristics of the internal combustion engine.
Locking the camshaft adjuster in a middle position is difficult, because the relative speed between the vane rotor and the drive part is very high when the camshaft adjuster is not activated and the pressure chambers are unpressurized. In this operating state the drive forces, which the crankshaft exerts on the drive part, cause the camshaft adjuster to shift to a retarding position. For improved engagement of the locking bolt the known locking device has a stepped locking well, the outer step of which takes the form of an elongated hole in the adjusting direction. In the event of a shift towards a retarding position when the pressure chambers are unpressurized, the locking bolt initially engages in the first step, which prevents the movement in the opposite direction beyond the middle position, in that one end of the elongated hole coincides with the middle position. At this end two steps are congruent, so that the locking bolt can engage in the second step, thereby locking the vane rotor in both adjusting directions. In one embodiment multiple steps may also be provided, which progressively restrict the freedom of movement of the locking bolt, the innermost step taking the form of a fixing well. The steps may furthermore extend in a peripheral direction to both sides of the innermost step.
The steps result in a relatively long radial adjustment travel for the locking bolt and hence a long spring travel of the locking spring. Furthermore, its free end is highly stressed when it is inserted and strikes against the limit of a step, so that it is subject to a large amount of wear. Over time this can result in a large amount of play between the innermost step, which serves as fixing well, and the locking bolt. The steps and the locking bolt are therefore hardened at the critical points.
DE 199 18 910 A1 discloses a camshaft adjuster which has two spring-loaded locking bolts which are spaced over the circumference of the vane rotor and which are displaceably guided either radially or axially in the drive part, and which, in the locking position, project into a locking well of the vane rotor. The locking bolts are hydraulically released in that a pressure acting in the pressure chambers acts on an end face of the locking bolts and shifts them into a release position against the force of the loading spring as soon as the pressure has attained a corresponding value. In this case the release bolts are cylindrical and are each subjected only to the pressure prevailing in either one of the pressure chambers acting in the retarding or advancing direction.
As a variant, the locking bolts take the form of stepped pistons, the small piston face being acted upon by the pressure of one pressure chamber and larger piston face by the pressure of the other pressure chamber. The resulting force generated by the pressures shifts the locking bolts into the release position.
Whilst one of the locking bolts interacts with a cylindrical recess designed as fixing well, which serves to lock the vane rotor in both directions of rotation, the other locking bolt interacts with an elongated hole, which extends over a circular path and one end of which defines the same locking position of the vane rotor, whilst the other end extends in the retarding direction.
If the one locking bolt is situated in the area of the elongated hole when the internal combustion engine is switched off, it can engage therein as soon as the pressure in the pressure chambers falls below a certain value. Once the locking bolt reaches the end of the elongated hole in the middle position of the vane rotor, the second locking bolt engages in the fixing well, so that the vane rotor is locked to the drive part in both directions of rotation. If the one locking bolt is situated outside the area of the elongated hole, for example in the area in which the greatest adjustment in the retarding direction is reached, oscillations when the internal combustion engine is started up will cause the vane rotor to move in the advancing direction, one locking bolt engaging in the elongated hole and preventing any return in the retarding direction, so that the other locking bolt can engage in the fixing well.
It is the object of the present invention to improve, by simple means, the release and locking of a camshaft adjuster also between the limit positions of its adjustment range.